In the Fourth Generation (4G) mobile communication system, a Reference Signal (RS) is utilized to perform data demodulation and channel sounding in an uplink of Single Carrier Frequency Division Multiple Access (SC-FDMA) for Long Term Evolution (LTE), for example.
The functions of the uplink RS include channel estimation required for performing coherent demodulation, channel quality sounding for uplink scheduling, power control, timing estimation, arriving direction estimation for supporting downlink beam forming, and the like.
The uplink RS in the LTE is mostly based on the Zadoff-Chu (ZC) sequence. The ZC sequence is also called GCL (Generalized Chirp-Like) sequence. This sequence is a non-binary unit amplitude sequence, which meets CAZAC (Constant Amplitude Zero Autocorrelation) characteristic. The CAZAC sequence is a complex value signal in the form of ejαk. The ZC sequence with a length being a prime number of NZC may be represented as:
            a      q        =          exp      ⁡              [                              -            j                    ⁢                                          ⁢          2          ⁢                                          ⁢          π          ⁢                                          ⁢          q          ⁢                                                                      n                  ⁡                                      (                                          n                      +                      1                                        )                                                  /                2                            +              ln                                      N              ZC                                      ]              ,where qε{1, . . . , NZC−1} is the root index of the ZC sequence, and n=0, 1, . . . , NZC−1, lεN. For simplicity, it is set that l=0 in the LTE.
The ZC sequence has characteristics as follows.
1. The ZC sequence has a constant amplitude, and still has a constant amplitude after DFT operation. The characteristic of constant amplitude limits the peak to average power ratio and the boundary and time flat interference on other users. Additionally, this characteristic simplifies the implementation when only the phase instead of the amplitude needs to be calculated and stored.
2. The ZC sequence with any length has ideal cyclic autocorrelation, which means that the cyclic shift correlation is a δ function.
Due to the ideal characteristics of the ZC sequence, the uplink RS has excellent characteristics as follows. (1) The amplitude is constant in the frequency field, which is for performing same activation on all of the allocated sub-carriers in the unbiased channel estimation. (2) The value of low Cubic Measurement (CM) is relatively low in the time field. (3) The autocorrelation characteristic is excellent, which facilitates accurate channel estimation. (4) The cross correlation characteristic is excellent, thereby reducing the interference of the RS transmitted over the same resource from other cells.
The uplink supports two kinds of RS. The first kind of RS is Demodulation RS (DMRS). This kind of RS mainly is used to perform channel estimation for uplink data transmission or signaling information transmission, and further to perform related detection. The second kind of RS is Sounding RS (SRS). This kind of RS mainly is used to perform channel quality measurement, thereby performing frequency selective scheduling for uplink.
In the LTE system, as shown in FIG. 1, a subframe is of 14 symbols numbered 0, 1, 2, . . . , 13, respectively. In the Rel.8/9 system, the SRS signal of the user can only be transmitted in the symbol No. 13. In the Rel.8/9 system, the DMRS signal of the user can only be transmitted in the symbols No. 3 and No. 10. The DMRS and SRS signals for uplink are time division multiplexed with data symbols. The DMRS for a specific User Equipment (UE) is of the same bandwidth as that of the Physical Uplink Shared Channel (PUSCH) or Physical Uplink Control Channel (PUSCH) transmitted by the UE, e.g. the PUSCH bandwidth in the whole cell bandwidth. Therefore, when the various bandwidths of the system are allocated to the various users (for FDMA), the DMRS for respective users are orthogonal to one another.
The SRS bandwidth for the user may be different from the bandwidth for data transmission. The SRS signal for the user is always transmitted over the last SC-FDMA symbol in the subframe, and the parameters of the SRS signal are notified by a high layer signaling. In FIG. 1, the UE transmits the SRS periodically over the last symbol in the subframe. In FIG. 2, the SRS signal is multiplexed through Frequency Division Multiplexing (FDM), Code Division Multiplexing (CDM), or Time Division Multiplexing (TDM). In the TDM, the eNodeB in the LTE requires s single SRS transmission from the UE or configures the UE to transmit the SRS periodically until the end. If a UE transmits the SRS periodically, the period may be 2, 5, 10, 20, 40, 80, 160, or 320 ms. In the FDM, the eNodeB may allocate the frequency resource with different bandwidth or the frequency resource with same bandwidth but with different location to the user for transmitting the SRS signal. The factors affecting the SRS bandwidth include the UE maximum power, the number of UEs supportable for detecting, and the detecting bandwidth required for benefit from uplink scheduling dependent on channel conditions. In the CDM, the eNodeB may also allocate the frequency resource with same bandwidth and with same location to different users. At this time, the different cyclic shift of the RS sequence may be used to implement the orthogonal sequence. That is, the correlation between a ZC sequence and any cyclic shift of the same sequence is zero. When the length of the channel impulse response is finite, different transmitters may use different cyclic time shifts of the same RS base sequence. As long as the cyclic shift is longer than the channel impulse response, the orthogonality may be maintained among the RSs.
Except for the TDM, FDM, and CDM above, there is no any other implementation for multiplexing of SRS resource in the current LET system.
In the LTE system, the transmission of the uplink signal of the user supports the mode of single antenna transmission or antenna selection transmission. Only a set of SRS resource is allocated to every user when the SRS resource is allocated.
In the LTE-Advanced system, in order to meet a higher uplink transmission rate index, it is required that the UE supports higher rank, e.g. rank1-rank4, of transmission on uplink. Thus, it is required that the UE is equipped with a higher number of antennas. For example, two antennas are equipped to support transmission of up to rank2, or four antennas are equipped to support transmission of up to rank4.
When the UE is equipped with a higher number of antennas to thereby perform higher rank of transmission, more SRS resources need to be allocated so as to perform detection and estimation on the channel of each of the antennas.
Therefore, additional methods should be adopted to further improve the capacity of the SRS.
The methods for improving the capacity of the SRS may be classified into two basic aspects. The first aspect is to improve the available resource of the SRS, and the other aspect is to improve the usage of the current SRS resource.
As described above, the parameters of the SRS signal are notified by a high layer signaling of the system, and then the SRS signal is activated and transmitted in a given period after the notification. This is a procedure for periodical transmission of the SRS signal. And, the deactivation of the SRS signal is notified by the high layer signaling of the system. Therefore, at present, the use of the SRS signal by the system, from the activation procedure to the deactivation procedure, has to be notified by the high layer signaling. The notification by the high layer signaling is a relatively slow procedure, and therefore the usage of the SRS resource of the system is relatively low.
The present invention is directed to employ the SRS resource with high efficiency, so that the system can support more users.